1. Field of the Invention
Recovery of oil from subterranean formations frequently involves displacing crude oil with a driving fluid, e.g., gas, water, brine, steam, polymer solution, foam, or miscellar solution. Ideally, such techniques (commonly called flooding techniques) would provide a bank of oil of substantial depth being driven to a producing well; in practice, that frequently is not the case. Oil-bearing strata are usually heterogeneous, some parts of them being more permeable to a driving fluid than other. As a consequence, channeling frequently occurs so that the driving fluid flows preferentially through zones depleted of oil (so-called "thief" zones) rather than through those parts of the strata which contain sufficient oil to make oil-recovery operations profitable. High permeability zones can also cause undesirable loss of drilling fluids when a well (e.g., water, oil or waste disposal) is being drilled. Misplaced casing perforations or casing leaks are another cause of channeling of the driving fluid through zones of high permeability in the subterranean formations.
A variety of fluid diversion techniques have been proposed in the prior art. Typically, a gel is formed in situ in zones of very high permeability, thus plugging them and causing fluid to flow through zones which originally were of lower permeability than those which have been plugged.
Polymers and other materials may be converted to gels in situ by appropriately adjusting the pH of solutions containing these materials. Typically, the materials used to effect pH change are organic esters, e.g. ethyl acetate. Such esters hydrolyze to form weak acids which can reduce solution pH. The rate of hydrolysis is a function of temperature, and numerous esters varying in reactivity can be used. Unfortunately, many of these esters cannot be used at higher temperatures above about 80.degree. F. In addition, since they form weak acids they are limited in the pH adjustment which they can achieve.
2. The Prior Art
U.S. Pat. No. 3,464,494 issued to McLaughlin relates to a method of plugging a formation with a solution of acid and sodium silicate having a pH of 1.5 or less to which fluoride is added; wherein the gelling time of the solution is controlled by varying the concentration of fluoride ion in the solution. The sodium silicate gelling time is controlled with fluoride ion obtained from any convenient source.
U.S. Pat. No. 4,203,492 issued to Watanabe discloses a method for acidizing siliceous materials contained in a high temperature formation wherein the injected aqueous fluoride salt solution forms hydrofluoric acid solution in situ when reacted with acid precursor. The fluoride salt must be capable of dissociating in situ to produce fluoride ions for generation of hydrofluoric acid. The fluoride salts disclosed include alkali metal salts of fluoroboric acid.
U.S. Pat. No. 4,732,213 issued to Bennett et al. discloses plugging of subterranean formations with gelled colloidal silica. Latent gelling agents are used to effect gelling,including materials which hydrolyze to release an acid or consume a base, e.g. hydrolyzable esters, acid anhydrides, sulfonates, organic halides or salts of a strong acid and a weak base.